1. Field of the Invention
The present invention generally relates to the field of adhesives, and more particularly to an adhesive mixture which enables electronic integrated circuit dies to be strongly attached to hybrid microcircuit substrates, and subsequently removed if necessary at a low enough temperature so as not to damage the substrates or adjacent dies.
2. Description of the Related Art
Thermosetting epoxies are widely used as adhesives for electronic applications. They are noted for their high adhesive strength to a wide variety of surfaces and materials. Their adhesive strength derives mainly from their highly polar molecular structure, which results in H-bonding and Van der Waals forces of attraction to surfaces such as alumina, ceramic, silicon, other epoxies, and even gold. Epoxies are therefore widely used, for example, for integrated circuit die attachment in hybrid microcircuits, component attachment in printed wiring boards, heat sink attachment, and ceramic substrate attachment inside hermetically sealed packages. Epoxy resins are generally of the diglycidyl ether of bisphenol A (DGEBA) types cured with a variety of catalysts or hardeners as is well known in the art. The reworking of components, once bonded with epoxy, has always presented a problem, and no ideal method has been heretofore discovered. Generally, heat and pressure are applied simultaneously to the device to be removed. When the epoxy reaches its glass transition temperature T.sub.G, which is usually 200.degree. C. or higher, it softens, at which point a laterally applied force effects its detachment. The entire circuit often must be subjected to high temperature (200 .degree. C. or higher), which risks damaging wire bonds or other temperature-sensitive elements of the circuit. Nevertheless, with the use of small die (less than 100 mils square) and moderately spaced die (greater than 200 mils chip-to-chip), epoxy-attached devices have been able to be reworked.
With the emergence of a new generation of very high density circuits, also referred to as hybrid wafer scale integration, or multichip module, the combination of high strength and reworkability becomes a critical issue. These new circuits utilize VLSIC (Very Large Scale Integrated circuits), VHSIC (Very High Speed Integrated Circuit), MIMIC (Millimeter Microwave Integrated Circuit), and other high-density dies that are approaching one inch square and are closely packed (less than 70 mils chip-to-chip) for high-speed applications. These large devices are also being assembled onto thin film multilayer interconnect substrates that have polyimide or another low-dielectric constant polymer as part of their structure.
Die-attach adhesives which meet all current standards and must also be removable will be needed for hybrid circuits where gate-arrays of approximately 600 mils square and chip-to-chip spacings of 70 mils are requirements. Other digital and analog circuits that will be fabricated using the HDMI (high density multichip interconnect substrate), also known as Hybrid Wafer Scale Integration, process will also require these adhesives.
Three problems arise in this environment.
1. The larger devices have a larger surface area, and therefore require greater force to remove.
2. The closer packed devices with small inter-chip spacings make the use of a heated tool to apply a shear force between devices difficult or impossible without damaging vicinal devices or wire bonds.
3. The extra force and temperature necessary to remove a die can damage the underlying organic dielectric, resulting in having to scrap a very expensive interconnect substrate.
Thermoplastic films and paste adhesives are also being investigated because they soften or melt. However, these suffer from two limitations for electronic applications.
1. Many of them must be dispensed from a solvent, and these solvents are difficult to completely remove, especially from underneath large area dies.
2. Most require very high temperatures (300.degree. C. or higher) to melt or soften and, depending on the extent of rework, such temperatures can be damaging to wire bonds and some devices.